Herrick, Morphology of Brain of Bony Fishes. 337 



infelicities of earlier technique. Several hours in cool 

 weather will not materially injure a fish brain in situ. The 

 medium which best fixes the brain elements in this case is a 

 modification of Fol's chrom-acetic solution. It is prepared 

 in a stock solution consisting of chi'omic acid of i per cent., 

 25 volumes; acetic acid of 2 per cent., 50 volumes; distilled 

 water, 2 c; volumes. This is the generally available fluid for 

 mammalian brains. For fishes it is reduced by the addition of 

 an equal volume of water. Large quantities of the fluid are 

 employed (20—30 volumes), and the brain left in it twenty to 

 twenty-four hours. Before setting the preparation aside the 

 utmost pains must be used to secure a natural and symmetrical 

 position of parts, in order to orientate sections properly. For 

 this purpose it is often of advantage to form a sling of tissue 

 paper suspended in the fluid to lift the specimen from the 

 bottom of the vessel and prevent flattening due to gravitation. 



Washing: — The brain, after fixing, is suspended in large 

 quantities of distilled water, which may be changed fre- 

 quently until the yellow color disappears (about one hour 

 usually suffices). 



Hardening: — The brain is passed successively through 

 50, 60, 70, 80 and 90 per cent, alcohol, remaining twenty to 

 twenty- four hours in each. It is of advantage in warm 

 weather to leave in c;o per cent, spirit ten hours and then 

 change to 55 per cent, for six to eight more before entering 

 60 per cent. The effect of sudden and great changes in con- 

 centration is to shrink and alter the cells. 



If the brains are to be cut at once they may be passed 

 from 90 per cent, alcohol to absolute after thirty-six to forty- 

 eight hours, but if they are to be preserved for some weeks 

 85 to 90 per cent, alcohol may be used. After remaining in 

 absolute alcohol for one dav or more the brain is ready for 

 imbedding. After a bath in turpentine lasting one to three 

 hours the tissue is placed in a mixture of turpentine and 

 paraffin, which is fluid at blood heat or a little higher tem- 

 perature, for an hour or two, 



